A very important step in the treatment of sewage by a wastewater treatment facility is to reduce undesirable levels of phosphorus from effluent wastewater before the effluent is passed on to a stream, lake, river or other terminal body of water. Wastewater treatment facilities often use biological treatment methods to remove undesirable substances such as BOD, ammonia and phosphorus, from wastewater. Biological treatment of the wastewater allows and encourages one or more types of bacteria normally present in wastewater to act upon the undesirable substances resulting in removal of these substances from effluent wastewater. So-called "enhanced biological phosphorus removal" (EBPR)is a biological method for reducing levels of phosphorus from wastewater. EBPR utilizes a biomass of microorganisms (bacteria) which can characteristically absorb large amounts of phosphorus from their environment during metabolic processes. For convenience, these microorganisms may be referred to as "Biological Phosphorous Removal Organisms" or "BPR organisms."
Some wastewater treatment facilities remove ammonia through a biological nitrification process that oxidizes ammonia. A problem with present EBPR methods in systems that also oxidize ammonia through the biological process of nitrification, is that the efficacy of the process is inhibited by the presence of compounds resulting from the ammonia oxidation, and in particular, nitrate and nitrite compounds (NO.sub.x --N). NO.sub.x --N compounds can prevent the selective growth of BPR organisms to the level needed for efficient phosphorus removal. NO.sub.x --N compounds are commonly found in nitrifying activated sludge treatment systems because ammonia, which is commonly found in wastewater, is nitrified during aerobic phases of wastewater treatment into NO.sub.x --N compounds. Thus, it is necessary to have an EBPR process for reducing the amount of phosphorus in wastewater which process includes a method for reducing the amount of NO.sub.x --N compounds returned in the activated sludge of wastewater treatment systems during treatment. Processes are known which use large internal recycle(s) of a treated mixture prior to clarification and large tankage (above the normal tankage requirements of a system) to achieve NO.sub.x --N reduction. However, the use of large internal recycle(s) and large tankage requires a modification of existing systems that greatly increases the capital cost and cost of operating a treatment system. Thus, it can be appreciated that it would be desirable to have an EBPR process which can achieve NO.sub.x --N reduction without the use of costly recycle(s) and increased tankage.